In recent years a two voltage based supply system has been introduced in the automotive world. Power consuming systems as for instance an electric turbo charger or an engine cooling fan can be supplied e.g. by a 48V supply voltage and other systems, as for instance a window lifter, are supplied by the classic 12V supply. One of the reasons for employing different supply voltages relates to the power losses, which scale with a power of 2 according to the Ohmic law: P=I*I*R.
If the supply voltage for a given system is increased by a factor of 4 from 12V to 48V, the current of that system scales down with a factor of 4 accordingly. However, the Ohmic losses via the supply wires scale down with a factor of 16, which saves energy and fuel consumption and reduces CO2 and NOx emissions.
As there are now two voltage domains in the car and there are systems, which are working on the 48V domain and others on the 12V domain, both domains need to allow a transfer of information signals between the systems of the different domains.
On the other hand, the voltage domains should not influence each other. This means a failure in the 48V domain should not influence systems in the 12V domain and vice versa. For that purpose, it is required that both domains on each system level (e.g. the engine cooling fan) be always galvanically isolated from each other. Only on a very few dedicated points in the car, the ground supplies of the 12V and the 48V domain are connected together.
On a given system level (e.g. an engine cooling fan) a common ground connection for both supply domains is not allowed. Both supply domains must strictly be separated from each other.
A Controller Area Network (CAN) or a Local Interconnect Network (LIN) bus (because of their nature and their specification) is always generated and handled in the 12V domain. A specification for these busses for the 48V is not planned, as the nodes would increase in cost.
This implies that, when a system in the 48V domain is controlled by a CAN bus, this system needs to have a galvanic isolation means to enable a signal exchange between the CAN bus of a 12V domain and the system as for instance engine cooling fan in the 48V domain. The galvanic isolation means must enable a signal transfer with a requested communication speed of, for instance, the CAN bus. On the other hand, information generated in the 48V domain, may need to be passed to the network in the 12V domain. This implies that the signal exchange over the galvanic isolation means must be bidirectional.
A prior art solution is illustrated in FIG. 1. A high level controlling unit 10 is shown comprising a communication bus 14, which is in this example a CAN bus. The CAN bus with its pins CANL, CANH is connected to a conventional transceiver device 15. The transceiver device 15 receives a supply voltage +12V as a signal VS via a C130 pin on a first connector 13 and a reverse polarity diode. Pin C131 of that first connector is the GND1 ground supply related to the +12V supply. The transceiver device further receives a supply voltage V1 of e.g. 5V via a voltage converter 12, e.g. a low drop voltage regulator. The transceiver device 15 is connected to a ground supply GND1 via the pin C131. The positive supply V1 and the ground supply GND1 are given also to an optocoupler 16.
The transceiver device transforms the signals CANL, CANH into the communication signals RxD and TxD. The transceiver device might have also other control signals e.g. a serial clock (SCK), slave device input (SDI), slave device output (SDO) and control signals (e.g. SCSN) in order to allow the transceiver device e.g. to be configured. The signals are given to the optocoupler 16, which transfers these signals in a galvanically isolated manner to the 48V domain. The transferred signals are fed to a microcontroller 17, which processes the signals further for a controlling means of the e.g. engine cooling fan. It should be noted that the microcontroller 17 can also send signals towards the optocoupler 16, so that the transceiver device 15 receives these signals. In other words, the signal transfer from the transceiver device to the optocoupler to the microcontroller is mostly bidirectional.
In the 48V domain a +48V supply is provided as C140 via a second connector 18 to a voltage converter 11, which can be for instance a DC/DC converter providing e.g. a voltage V2 of +5V to a microcontroller 17. V2 is also provided to the optocoupler 16. Microcontroller and optocoupler are supplied with a ground supply GND2, which is provided via pin CI41.
It can be noted that the optocoupler 16 has two separate supply pins (V1, V2) and two separate ground pins (GND1, GND2). Using an optocoupler, a galvanic isolation between the two supply domains is possible. The optocoupler may be replaced by e.g. an inductive based signal transformer, which also realizes a galvanic isolation. Other alternatives are available.
However, the CAN bus is a high-speed bus with data rates up to 2 MBit/s. Especially in the automotive environment optocouplers might be too slow. As an optocoupler is an additional device, also additional cost and space issues are involved.
WO2012/159168 discloses an USB driver, galvanically isolated with the signal transfer in a full differential approach. The hardware efforts and thus costs might be high. Due to noise there may be robustness issues in the automotive harness.
WO2014/029585 and US2009/206960 show a full differential approach wherein four capacitors are used for a galvanic isolation of one signal. Consequently, the required hardware effort and thus cost are relatively high.
In US2013/279550 a method of communicating data values over a three-conductor interface is presented. Different data values are transmitted by generating and transmitting three respective signals to a receiver using three respective conductors. The first signal is maintained as a set voltage level. The second signal is alternated between a high voltage and a low voltage according to a carrier frequency. The third signal is alternated between the high and low voltages and is out of phase with the second signal. Signal disturbances related noise, especially in automotive harness, however has a negative effect. Additionally, the hardware efforts and thus costs are high.
A solution of a somewhat different scope is found in US2008/180226, where two CAN networks are separated with a galvanic isolation barrier.
Hence, there is room for improvement. In particular, there is a need for a solution wherein the optocoupler is removed and the galvanic isolation means and the signal transfer over it are realized in a different way.